Japanese Patent Application Laid-Open No. 5-152638 (Patent Document 1), for example, discloses a piezoelectric sheet with a piezoelectric constant d14, which can be obtained by stretching a molded product of a polylactic acid. More specifically, as shown in FIG. 16, assuming that a piezoelectric sheet 11 made of a polylactic acid has electrodes formed on upper and lower surfaces in the figure and is stretched in a “3” axis direction, stain will be produced in a “4” axis direction which is the rotating direction of “1” axis, when an electric field is applied in the direction of the “1” axis which is a normal direction with respect to the electrode surfaces.
This piezoelectric sheet made of a polylactic acid is used to provide a unimorph or bimorph structure, thereby allowing piezoelectric elements such as piezoelectric vibrators to be obtained for use in speakers, microphones, etc. While Patent Document 1 discloses these applications, it merely mentions that “when the molded object stretched is a film, the film is cut into an appropriate size to serve as a product” (section [0018]) and includes no specific disclosure on the method for constituting the piezoelectric elements.
On the other hand, the stretched sheet has the problem of tendency to tear along the stretching direction. In this case, if biaxial stretching is carried out, the tendency to tear will be alleviated to some extent. It is to be noted that the substantially uniaxial stretching by four to six times has to be obtained in order to keep high piezoelectricity. Therefore, in the case of biaxial stretching in the X axis direction and the Y axis direction, the ratio of the stretching in the Y axis direction to the stretching in the X axis direction has to be set to 1:4 to 1:6. For example, when biaxial stretching is to be carried out in the X axis direction, the stretching by 8 times or more has to be carried out in the Y axis direction.
However, even when the biaxial stretching is carried out as described above, the problem of the tendency to tear will still remain while the problem is alleviated to some extent. In particular, in the case of using the piezoelectric sheet for actuators, speakers, etc., the problem of the tendency to tear is serious because the piezoelectric sheet always has relatively high stress caused.
Further, this problem is encountered not only for piezoelectric sheets made of a polylactic acid, but also generally for piezoelectric sheets which have a main constituent made of a helical polymer with a chiral molecule as a unit and exhibit piezoelectricity when they are stretched.
Patent Document 1: Japanese Patent Application Laid-Open No. 5-152638